Invertebrate of the Week returns with this emerald-colored stunner. Meet Elysia chlorotica, often hailed as a “solar-powered marine sea slug” and affectionately named the “Eastern Emerald Elysia.”

E. chlorotica inhabits the waters along the Atlantic Coast of North America (from Nova Scotia to Florida) and along the Gulf Coast of the United States. It can be found inhabiting saline marshes, intertidal pools, and brackish canals at a depth between 0 – 0.5m. This penchant for maintaining a shallow depth range within the reach of sunlight provides a clue to what gives this beautiful sacoglossan its a stunning emerald green color: chlorophyll. The very same photosynthetically-active pigment that allows plants to derive their energy needs from the sun is also responsible for E. chlorotica striking coloration.

Elysia chlorotica. Photo: New Scientist (http://goo.gl/HGSLlG)

But E. chlorotica’s relationship with chlorophyll appears to extend beyond looking fashionable. Research by scientists like Sydney ‘Skip’ Pierce suggests that this slug may be capable of sequestering the chloroplasts within its body in a such a manner as to permit them to continue photosynthesizing; thus providing the slug with a steady supply of food.

Elysia chlorotica. Photo: Nature (http://goo.gl/xzA9TD)

After emerging from their larval stage, young E. colorata’s set-out to consume large quantities of Vaucheria litorea algae. The slugs pierce the algal filaments and suck out the soup of algal cytoplasm within. This liquid diet passes into the digestive tract where the chloroplasts – those double-membrane organelles that contain the algae’s chlorophyll pigments – are isolated and sequestered into vacuoles along branches of the digestive tract. According to Prof. Pierce’s hypothesis, eventually enough chloroplasts are ingested and sequestered to allow the slug to produce enough products of photosynthesis to provide a steady supply of food.

But for this method to work, the story can’t stop there…

You might recall from basic biology courses that chloroplasts have their own DNA, separate from the organism they are found within. When inside their native V. litorea algae, the chloroplasts rely on a combination of proteins encoded in their own genome and on proteins encoded by the genes of the algae itself. Without the two genomes working in concert, the chloroplasts are unable to function. One would think that once the algal chloroplasts were translocated into their new E. colorata host, they would lose access to the vital proteins produced by their former algal home and would subsequently become dysfunctional.

Here’s where things get interesting. Since the chloroplasts appear to be fully operational, they must be getting those vital proteins from somewhere but the mechanism by which this takes place remains a mystery. Two possibilities put forth propose that the slug, as a species, incorporated algal DNA into its own genome at some point, either through horizontal gene transfer or through a viral vector. Both possibilities, however, have their pitfalls and neither has been sufficiently supported by the studies thus far.

Elysia timida. Photo: Sven Gould/Jan de Vries

Other researchers like Sven Gould and graduate student Christa Gregor have offered a different explanation. Based on observations from two similar species, Elysia timida and Plakobranchus ocellatus, they believe that the chlorophyll may just be stored as a fat and protein rich food reserve to be used in times of need. This might explain why the slugs are able to endure prolonged periods of starvation and become progressively paler when they’re unable to graze on algae.

Perhaps the answer is in some combination of the two possibilities, with the slugs using photosynthesis in concert with ‘chlorophyll as food-storage reserves’. I, personally, can’t help but wonder if the proteins necessary for photosynthesis could be obtained through ingesting a steady supply of protein rich V. litorea cytoplasm. Any marine algal specialists, biochemists, slug geneticists, etc. reading this want to weigh in?

Whatever the case, I’m sure the conclusions of this ongoing research effort will be nothing short of fascinating.